1. Field of Invention
The present invention relates to a photomask, and more generally to a photomask for imaging quality improvement caused by lens aberration under extreme illumination.
2. Description of Related Art
In recent years, the trend in the development of semiconductor is toward miniaturization of circuit devices. Photolithography plays a key role in such a miniaturization development for semiconductor industry because it determines the critical dimension (CD) of a thin film pattern related to semiconductor device structures. Thus, the accuracy of a photomask pattern transfer is very important. If the pattern transfer is inaccurate, the tolerance of the critical dimension on the chip can be affected and the resolution of the exposure can be reduced.
However, due to the absorption of energy for lens when light passes through the scanner, the lens is subject to uneven thermal expansion because diffraction patterns are not distributed uniformly. Images deform due to lens aberration, and the imaging quality is accordingly reduced. The above-mentioned problem becomes more serious when the illumination uses a strong off-axis illumination (OAI) such as a dipole illuminator.